Can I join GDI as a teenager?

If the TOS says 18, it's 18. Nothing to debate. As for taxes ... you pay them if you owe them. Again, nothing much to debate.

First thing I thought of was command & conquer.

Yeah if they report to the IRS you can't really get around lying about your age like you could with Ebay.

Nothing against GDI but I have never met anyone that has actually enjoyed the success and or achieved the goals suggested, as another poster opined, there are a lot of really great learning resources right here.

After all The Warrior Forum, is where everyone ends up after they try everything under the sun that promises sunshine and lollypops but fails to deliver...

They end up here because they loose all of their hard earned money trying everything to get to a position where you are right now.

Take this opportunity to learn from all the mistakes that most of us made in the past.

If you really want to make money, you will need to do it yourself, contrary to popular belief's and or other Pie in the Sky Barnum Baily, politics most people do not make money online because they try to earn commission,
(working for someone else)
They try to make it by referring others to the same program,
(working for someone else and ripping people off)
They try to figure out how they can provide services that other marketers want...
(requires skill and talent)

Then there are those that develop unique quality products because they have talent, they have creative genius and they can see around the corner...

Forget about working for someone else...

Save your money and learn as much as you can and you will not only save a ton of money on your car insurance, but you will also find that skipping all that trash they sell out there will help you in the long run more than you can imagine.

Hi Sky Reacher, super hero in the making, just wanted to drop my congrats for your motivation and interest in living a life extra ordinary, you have come to one of the right best paces to learn all you need.
In fact you'll discover, like I did, that most of the world's experts and even owners of powerful high achieving website businesses, still come here to give and take, years after they made enough money get out of the rat race (swapping enough time for a wage to stay alive, wherein the game is to be more efficient at it than the next person and have enough money left over after the bills are paid to have a life, except, oops, now you have no time, do you?!.)
Anyway, welcome, you're in the right place, in yourself and on this website, all the best, BR88

Tax is tax, you have to report what you earn.

As for your age, you must be 18 to join - but I believe that one easy way to get around it would be to ask your parents to sign up under their name.

I've had no experience with GDI, but some companies, if they find out that you've broken their terms, may be able to withhold any accrued money during a defined period.

Don't go with them. I suggest that you don't go with them. Mostly they are scams.